Constellation mapping is a digital modulation technology. A constellation mapping process is mapping a finite field “bit” sequence carrying digital information to a “symbol” sequence suitable for transmission. A value space of each symbol may be a one-dimensional real number space, or a two-dimensional real number space (that is, a complex number space). The constellation mapping includes two elements, that is, a constellation diagram and a constellation point mapping method. The constellation diagram represents a set consisting of all values of output symbols of the constellation mapping. Each point in the constellation diagram is corresponding to a value of an output symbol. The constellation point mapping method represents a specified mapping relationship from an input bit (sequence/group) to a constellation point, or a specified mapping relationship from a constellation point to a bit (sequence/group). Currently, the most common and widely-used constellation diagrams mainly include pulse amplitude modulation (PAM) of the one-dimensional real number space, quadrature amplitude modulation (QAM) of the two-dimensional real number space, and phase shift keying (PSK) modulation.
As a communications system raises an increasingly higher requirement for a transmission rate and spectral efficiency, before performing constellation diagram mapping on a data bit stream by using a QAM technology, a transmit end in an existing communications system usually codes some or all bits in the data bit stream by using a forward error correction code technology. Specifically, the transmit end may actually divide either a coded bit stream or an uncoded bit stream in the data bit stream into a corresponding in-phase (I for short) component and quadrature (Q for short) component, and perform constellation diagram mapping on either the coded bit stream or the uncoded bit stream according to the I component and Q component corresponding to either the coded bit stream or the uncoded bit stream, to obtain a coded constellation diagram and an uncoded constellation diagram. The transmit end further integrates the coded constellation diagram with the uncoded constellation diagram in a constellation diagram merging manner, to obtain a constellation diagram corresponding to the data bit stream. QAM of different orders may be used according to different conditions of a network system. The QAM of different orders is 2n QAM, and different orders are specifically represented by using different integers n. Greater n indicates higher spectrum utilization. If n is an even number, the constellation diagram is a square constellation diagram, for example, 4QAM or 16QAM; or if n is an odd number, the constellation diagram is a rectangular constellation diagram, for example, 32QAM, 128QAM, or 256QAM. A value of n corresponding to the QAM of different orders is a length corresponding to the data bit stream. Because two signals corresponding to the rectangular constellation diagram, that is, an I component and a Q component, are corresponding to asymmetrical powers, and energy at the constellation point is excessively large, the rectangular constellation diagram further needs to be shaped, to obtain a cross-shaped constellation diagram. Correspondingly, a receive end needs to perform decoding and QAM demapping, to obtain the corresponding data bit stream. However, before performing decoding, the receive end further needs to learn bit soft information indicating a bit value probability, that is, a maximum log-likelihood ratio (LLR).
In the prior art, in a cross-shaped constellation diagram corresponding to a rectangular constellation diagram 128QAM obtained according to an odd number of bits, values of coded bits corresponding to one I component but different Q components or one Q component but different I components are different. Therefore, when an LLR required for decoding a bit is to be calculated, quite fine area division needs to be performed on the cross-shaped constellation diagram. As a result, the LLR calculation is relatively complex.